Most of marine aquaculture is not sustainable and is actually taking away protein from the human population. Around 1990, the wild capture fisheries leveled off at 95 million metric tons per year while aquaculture had an almost exponential rise. Most of aquaculture is freshwater fish. Recirculating land-based systems need huge infrastructure and have a very high energy demand with a high carbon footprint. At the medium level, there are net pens for sea bass and salmon and at the low level, there are passive rope cultures of mussels and algae that require no additional feeding. The long food chains in the ocean mean that if we want to eat tuna, for example, we need to feed them with other fish – called forage fish – caught at rate of 20 million tons per year. This is lost protein that could be available for human nutrition; we could replace this fish-derived protein with plant-derived protein or raise more omnivorous fish like tilapia or carp. We have to have seriously overfished fish stocks to make aquaculture profitable; also, aquaculture requires a lot of space and requires the conversions of land. A solution could be farming down the food web, which means that we should tackle lower tropic levels for aquaculture, like filter feeders like oysters.
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